The art and science of the design of papermaking machines have made significant advances in the last twenty years. The direction of these improvements has been towards increased efficiency in the manufacture of paper by increasing the speed at which the web is formed and by increasing the width of the web. With increased machine speed there is a need--particularly in the pressing section of a papermaking machine--to increase the time the paper takes in moving through press nips. This is most effectively accomplished by utilizing a shoe with a concave surface opposite a backing roll. In order for the web to move through the nip without frictional resistance a blanket is passed over the shoe. The blanket is lubricated and cooled by a supply of oil or other lubricant which is forced between the blanket and the shoe. One type of press utilizing a shoe is the so called open Extended Nip.RTM. press of the type manufactured by Beloit Corporation of Beloit, Wis. In this type of press, the lubricating blanket is supported on a plurality of support rolls which direct the blanket through the nip between the backing roll and the shoe in a way similar to the way in which conventional felts are directed.
In some circumstances open Extended Nip presses result in leakage of oil onto the felts and the web as they transit the nip. In other circumstances it is desirable to place the shoe above the backing roll. In this position the lubricating oil has a tendency to drain down onto the web being pressed. This has led to the development of the so called closed Extended Nip press. A closed Extended Nip press has a blanket the ends of which are extended beyond the nip and the shoe and sealed to a circular head. Sealing the ends of the blanket solves the problem of lubricant leakage. As the blanket moves through the nip it forms a cardioid shape as the circular blanket is deformed between the shoe and the backing roll. Thus the blanket transitions from a cardioid shape at the nip to the circular shape required by the mounting to the heads which contain the lubricant. The transition region of the blanket between the cardioid shape and the circular shape rotates as the blanket rotates through the nip. The repeated flexure of the blanket eventually causes the blanket material to fail. This failure of the blanket can be a significant source of maintenance down time, which, owing to the high capital investment in a papermaking machine, is a significant contributor to costs in the production of paper.
The motion of the blanket through a closed Extended Nip press results in a gradual lengthening of the blanket in the cross machine direction. In a typical Extended Nip press one head or end of the blanket is held fixed and the other is allowed to float. The free end is thus able to move laterally to accommodate the growth in the blanket. A gradual lengthening of the blanket takes place throughout the portion of the blanket which passes through the nip. Thus the end of the blanket which is held fixed experiences little or no lengthening, and the region of the blanket which is fatigued by the transition between the circular end heads and the cardioid shape remains fixed with respect to the shoe.
One approach to achieving a longer life of the closed nip blanket is to move the fixed side of the blanket by, for example, moving the position of the fixed side of the blanket. One known approach is to install spacers against a fixed stop thus moving the fixed end of the blanket so the fatigue-stressed region is shifted with respect to the fixed end. The difficulty of adding and moving spacers makes this approach less than completely satisfactory.
What is needed is a method and apparatus for extending the life of a blanket in an Extended Nip press.